This invention relates to a new and improved method and apparatus for cooling and dehumidifying air and more particularly to a hybrid double absorption system employing a liquid desiccant solution and capable of operating at low heat source temperatures between 55.degree. C. (131.degree. F.) and 80.degree. C. (176.degree. F.).
Much effort has been expended in utilizing solar energy as an energy source for cooling of buildings and other enclosures since the available solar energy and the required cooling are generally in phase. The methods developed to date generally involve one of three cycles: the absorption refrigeration cycle; the rankine/vapor compression cycle and the dehumidification/evaporative cooling cycle using a solid or liquid desiccant. Although a few have become commercially available, none have been able to efficiently employ low grade thermal energy below 80.degree. C. (176.degree. F.). This is particularly unfortunate since flat plate solar collectors become relatively inefficient and expensive when designed for use at or above 60.degree. C. (140.degree. F.).
At least one absorption-refrigeration cycle for solar applications has employed a LiBr-H.sub.2 O absorption cooling system. However, it is believed that none of the absorption-refrigeration cycles is able to efficiently employ low-grade thermal energy below 80.degree. C. (176.degree. F.).
The Rankine/vapor compression cycle is essentially a solar powdered Rankine cycle engine combined with a conventional vapor-compression cooler. This system has the advantage that electricity can be generated if a motor/generator is used to interface the engine with the cooling cycle thereby making the cooling cycle part of a total solar powered system. However, it requires even higher collector temperatures than absorption-refrigeration cycles to achieve good efficiency and more expensive concentrating collectors. Since solar collectors comprise a major portion of the capital investment in a solar cooling system, it is not considered economical to use the Rankine/vapor compression cycle for small residential applications and it is also not applicable to low grade energy resources.
Solid desiccant cooling/dehumidification systems can be powered by low cost flat plate collectors. However, such systems require a large volume of solid desiccant and also entail significant operating costs for the parasitic system of blowers to circulate both the air to be conditioned and the warm air for regeneration. The air must be circulated through large beds of solid granular material, using a considerable amount of blower power. The overall system coefficient of performance of solid desiccant systems is, therefore, considerably less than that of the absorption cooling system. Additionally, the average delivery air enthalpy from a solid desiccant system is generally higher and thus the solid desiccant system has to run longer to satisfy the same load than does the absorption system, thus using even more power, which makes it less attractive for solar applications.
Liquid desiccant systems have also been employed. Generally speaking, as compared to solid desiccant systems, liquid systems require less blower power, produce drier air and have higher system coefficients of performance (C.O.P.). However, such systems have until now been unable to function efficiently with low-grade thermal energy provided at less than 80.degree. C. (176.degree. F.). For example, U.S. Pat. No. 4,205,529 reveals a hybrid air conditioning system that combines a solar powered lithium chloride dehumidifier including a regenerator with a lithium bromide absorption chiller. However, for efficient operation of the system a temperature source of at least 180.degree. F. (82.2.degree. C.) and preferably 200.degree. F. (93.3.degree. C.) is required in the absence of compressors in the absorption chiller.
Table 1 provides an overview of the various methods employed to date using low grade thermal energy:
TABLE 1 ______________________________________ Temperature Level Estimated Method Required Applications C.O.P.* ______________________________________ Rankine/vapor &gt;100.degree. C. Large scale 0.8 compression Absorption LiBr/H.sub.2 O Small or Single effect &gt;80.degree. C. large scale 0.5-0.8 Small or Double effect &gt;130.degree. C. large scale 0.9-1.5 Small or Absorption NH.sub.3 /H.sub.2 O &gt;120.degree. C. large scale 0.4-0.7 Dehumidification Small or Solid desiccant &gt;60.degree. C. large scale 0.3-0.5 Small or Liquid desiccant &gt;60.degree. C. large scale 0.3-0.5 ______________________________________ *Based on thermal input
As can be readily seen, the lowest temperature level attained 60.degree. C. (140.degree. F.) results in a coefficient of performance of 0.5 at best, while the highest coefficient of performance (0.8 ) requires a source temperature of at least 80.degree. C. (176.degree. F.).
The prior art devices suffer from other defects as well, including extensive equipment requirements, a need for two working fluids and lack of versatility in meeting changing ambient conditions and system cooling requirements.